1. Field of the Invention
This invention relates to a method of making a nitride-based compound semiconductor crystal and method of making a nitride-based compound semiconductor substrate.
2. Description of the Related Art
Nitride-based compound semiconductors such as GaN have a wide bandgap and are of direct transition type and, therefore, they attract attention for a material to emit ultraviolet to blue light.
In fabricating a light emitting device of nitride-based compound semiconductor, hetero-substrates such as sapphire have been used. However, the hetero-substrates have problems that a high dislocation density is generated in the hetero-epitaxial growth and the device fabrication thereof is complicated.
To solve the problems, self-standing substrates of nitride-based compound semiconductor are actively developed. A typical example thereof is a method that a GaN thick film is grown on a hetero-substrate such as sapphire by HVPE (hydride vapor phase epitaxy) and then the hetero-substrate is removed to obtain a self-standing substrate (e.g., JP-A-2003-178984). In this method, a void-containing layer functions as a strain buffering layer so as to buffer a strain caused by a difference in lattice constant or thermal expansion coefficient between the underlying substrate and the group III nitride-based compound semiconductor layer grown thereon. By the method, a substrate of group III nitride-based compound semiconductor can be obtained which offers a reduced defect density and a good crystalline quality without warping. Further, the self-standing substrate thus obtained can be easily separated. Based on the method, GaN substrates with a reduced dislocation have begun to be commercially available.
Another method for obtaining the self-standing substrate is also proposed. In this method, a metal thin film of Ti etc. with a catalytic activity to promote the decomposition of GaN is formed on the surface of a GaN underlying layer, and then it is thermally treated in NH3 containing atmosphere to form a mesh-like TiN layer on the GaN layer and to generate an inverted cone-shaped void corresponding to the mesh space in the GaN underlying layer. Then, GaN is laterally grown on the TiN layer, and a GaN layer thus grown is separated by using solution of fluorinated acid and nitric acid. Thus, a self-standing substrate can be obtained with a reduced dislocation density as low as 107 cm−2 and without warping (e.g., JP-A-2002-343728).
However, the GaN substrate made by HVPE has a problem that it is very costly. Therefore, it is little available in the market. The high cost is mainly caused by that each of the GaN substrate needs multiple steps in growth thereof. In general, the growth steps include the hetero-growth on a hetero-substrate such as sapphire and GaAs. Therefore, the underlying substrate needs to be carefully prepared so as to prevent the generation of dislocation during the GaN crystal growth.
For example, the preparation of the underlying substrate is conducted such that a GaN thin layer of about 1 μm in thickness is formed on a sapphire substrate by MOVPE (metalorganic vapor phase epitaxy), a stripe mask of SiO2 with a width of a few micrometers is formed thereon by fine photolithography, and a GaN thick film of about 500 μm in thickness is formed by HVPE on the underlying substrate. Then, the GaN thick film is separated from the sapphire substrate by laser separation method etc. Then, by polishing both faces of the film, a GaN substrate can be made at last. Thus, the steps are repeated in making each of the GaN substrate.
Since it is thus made through the multiple steps, the product yield of the GaN substrate is very low. Especially, there are problems in the step of growing the GaN underlying layer and forming the mask, and in the step of removing the hetero-substrate.